Charring Model: Transport within the structure is based on a homogenization theory description of porous media. Volume fractions of each constituent are tracked, i.e. gas, char, resin, fiber. The transport of gas and species mass fractions are modeled using Darcy’s and Fick’s Law. The temperature field is solved for assuming thermal equilibrium among the constituents. A Galerkin finite element formulation using high-order Lagrange interpolating polynomials for hexahedral elements is used to solve the transport equations. An new FE expansion algorithm is introduced to account for the swelling of carbon epoxy [1].

Flow Solver: Compressible form of the Navier Stokes equations with multicomponent species transport and an energy equation for a reacting system are solved. The flow solver is based on a finite volume formulation using the AUSM family of flux splitting schemes.

Coupling Algorithm: A new coupling algorithm, with four

primary components, is developed and implemented [2].

New Insights on Flame Spread Over Charring Materials using High Fidelity Numerical Modeling and SimulationMatthew T. McGurn and Paul E. DesJardin

Department of Mechanical and Aerospace Engineering University at Buffalo, State University of New York

New Insights on Flame Spread Over Charring Materials using High Fidelity Numerical Modeling and SimulationMatthew T. McGurn and Paul E. DesJardin

Department of Mechanical and Aerospace Engineering University at Buffalo, State University of New York

IntroductionIntroductionComposite materials are being used at an increasing rate in applications including aerospace vessels and other transport vehicle due to the high strength to weight ratio, corrosion resistance, and ease of fabrication. One of the safety challenges to application of composite materials is their susceptibility to fire. When exposed to fire, composites degrade, releasing volatile gases, and producing char, thereby reducing structural integrity. This study concerns the development of a numerical methodology to conduct conjugate heat and mass transfer simulations of burning composite materials leading to ultimate material failure.

ModelingModeling

ReferencesReferences[1] McGurn, M. T., DesJardin, P. E., and A. B. Dodd, "Numerical simulation of expansion and charring of carbon-epoxy laminates in fire environments," Int. J. Heat Mass Transfer, vol. 55, pp. 272-281, 2012. [2] McGurn, M., K. Ruggirello, and P. E. DesJardin. "An Eulerian- Lagrangian Moving Immersed interface Method for Simulating Burning Solids." J. Comput Phys. (2012).[3] Quintiere, J.G., Walters, R.N., Crowley, S., "Flammability Properties of Aircraft Carbon-Fiber Structural Composite," U.S. Department of Transportation, Federal Aviation Administration: Washington, DC. 2007

The model is based on the extensive property data given by the study of Quintiere, Walters and Crowley (QWC) [3]. Quintiere and colleagues developed a complete set of properties for carbon-fiber composites (thermal properties, kinetics of degradation, and heat of decomposition). Validation of the numerical models are compared to the well documented coupon level, Time-to-ignition, and one-sided heating experimental results from QWC.

Carbon EpoxyReproduced from QWC

Coupon Scale: Coupon scale simulations showing (left) instantaneous snapshots of temperature (solid lines) and reaction progress α (dashed lines) at t = 775, 1775 and 2025s for a 10 C/min heating rate and (right) comparison of predictions of solid mass fraction (solid lines) to TGA data from Quintiere et al. (symbols) and their curve fit using a first-order Arrhenius rate model. The overall agreement is reasonable - demonstrating that the Arrhenius kinetics are are properly incorporated into the framework.

Time-to-Ignition: The time-to-ignition is estimated based upon a critical mass flow calibrated from experimental data. It is assumed that this critical mass flow is sufficient in providing fuel to produce a flame off the surface. Simulations are

Case Critical Mass Flux

Open 2.40 g/m2·s

Sealed 3.35 g/m2·s

performed for both open and sealed back boundary conditions. Overall good agreement is achieved.

One-Sided Heating: The heat release rate (HRR) was calculated for cases with both sealed and open right boundary. Comparing simulation cases without (w/o) and with (w/ ) the expansion model shows that accounting for volumetric expansion processes extends the burnout time by at least 50% for all cases resulting in much better agreement to the experimental data. Since the simulations with the expansion model with open and sealed appear to bound the HRR data, it is reasonable to assume that the actual boundary from the experiments lies between these limits.

The final product volume and mass fractions are well predicted with the overall error for V/Vo, Vg/Ve & Yrc less than 15%.

Fully Coupled Simulations: For most charring materials self-sustained burning will not occur unless a separately imposed heat flux is applied. This heat flux is in addition to that from the flame. The minimum heat flux to sustain burning is the critical heat flux. QWC experimentally found the critical heat flux to be 14.2kW/m2. Simulations results are consistent with this value.

ResultsResults

SummarySummaryNovel contributions…

• Unique flow/structure coupling algorithm & framework for modeling flame spread

• Thermal model for charring materialsFindings…

• Good overall agreement between model predictions and experimental data for time-to-ignition, burnout time, HRR & critical heat flux for flame spread

• Neglecting swelling results in under-prediction of flame burnout and overall energy release

SponsorsSponsors

Flow Field Carbon Epoxy Panel

NSF Award: 103328CBET Program Director: Arvind Atreya